On Wed, Jun 01, 2022 at 01:18:52PM +0100, Joao Martins wrote: > > So having safe racy reading in the kernel is probably best, and so RCU > > would be good here too. > > Reading dirties ought to be similar to map/unmap but slightly simpler as > I supposedly don't need to care about the pte changing under the hood (or > so I initially thought). I was wrestling at some point if test-and-clear > was enough or whether I switch back cmpxchg to detect the pte has changed > and only mark dirty based on the old value[*]. The latter would align with > how map/unmap performs the pte updates.
test-and-clear should be fine, but this all needs to be done under a RCU context while the page tables themsevles are freed by RCU. Then you can safely chase the page table pointers down to each level without fear of UAF. > I am not sure yet on dynamic demote/promote of page sizes if it changes this. For this kind of primitive the caller must provide the locking, just like map/unmap. Effectively you can consider the iommu_domain has having externally provided range-locks over the IOVA space. map/unmap/demote/promote must run serially over intersecting IOVA ranges. In terms of iommufd this means we always have to hold a lock related to the area (which is the IOVA range) before issuing any iommu call on the domain. Jason _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
